Pavement marking or pavement striping is conducted by applying paints, resins, reflective materials, and/or reflective media onto streets, roads, or parking lots, These markings serve a variety of purposes: they demarcate roadway lane boundaries, identify where it is appropriate to pass cars traveling in the same lane of traffic, identify where pedestrians are permitted to cross a street or intersection, identify where it is or is not appropriate to park a vehicle in a parking lot, and indicate restrictions and permissions on lane usage. These markings must be clearly visible in both daylight hours and in the less than optimal conditions, such as during twilight or evening hours. Moreover, these markings must be visible even under wet conditions and be able to withstand constant wear from vehicle and pedestrian traffic.
Although advancement has been made to increase the visibility of paints, current standards find the reflective quality of paint less than adequate. One solution to increase the reflectance quality of paints is to incorporate a reflective material within the paint as it is applied to the pavement surface. This technique may also be useful for resins (e.g. thermoplastics or epoxies) and tapes which may contain reflective materials called retroreflectors. Retroreflectors are devices that send light or other radiation back where it came from regardless of the angle of incidence, unlike a mirror, which does so only if the mirror is exactly perpendicular to the light beam. Retroreflectors produce the effect of retroreflection (also called retroflection) and possess retroreflectivity characteristics. One such retroreflector is a highway marking sphere, e.g. a glass bead, having a refractive index of at least 1.5. Each marking sphere behaves like a spherical lens reflecting multiple angled incident light back to the motorist. When light from a vehicle headlight enters the marking sphere, it travels through the marking sphere, strikes the pavement marking material, and is reflected back toward the source from which the light originated, i.e., the driver of the vehicle. In this manner, in the pavement marking material, e.g. the paints, tapes or resins, reflectance qualities are increased and make it easier for drivers to see these pavement markings at nighttime.
Incorporation of marking spheres into paints and resins while maintaining the retroreflectivity of a highway marking has its challenges. With paints and resins, marking spheres can be mixed into the paint or resin before application, or the marking spheres can be applied just after the paint is applied to the highway. Of these incorporation techniques, the latter technique is generally preferred because the marking spheres are adhered to the pavement marking material, but not embedded completely below the surface of the pavement marking material. This application technique allows the marking spheres to be present at the surface of the pavement marking material where retroreflectivity of the applied highway marking can be immediately utilized.
That is to say, other techniques have certain disadvantages. For example, marking spheres mixed into the paint or resin before application tend to have marking spheres within the layer of paint or resin as it is applied on the pavement surface. These embedded marking spheres are not easily removed from the pavement marking surface, but also cannot be immediately utilized. They can be subsequently utilized after the top layer of pavement marking material is worn away from vehicle traffic or weather.
A typical device to dispense marking spheres just prior to application is a marking sphere dispenser. A marking sphere dispenser is typically located on a movable vehicle that also carries the paint or resin applicator, so that an appropriate quantity of marking spheres are dispensed onto the width of the pavement marking in accordance with predetermined marking characteristics. The vehicle carrying the marking sphere dispenser is generally moving while the pavement marking materials and marking spheres are applied to the pavement surface. Therefore, if the marking spheres are simply dropped directly onto the pavement marking material as the pavement marking material is applied, the relative velocity of the marking spheres approximates the velocity at which the vehicle is moving over the pavement.
The relative marking sphere velocity is responsible for a phenomenon called marking sphere rolling. It is typically seen that applying marking spheres from a vehicle traveling less than about 8 mph does not result in any significant marking sphere roll. At these speeds, the amount of road surface covered in a day is meager. There is a desire therefore to increase the application speeds, but application speeds above 8 mph are problematic in that these speeds impart a significant relative velocity to the marking sphere. The relative velocity at which the marking spheres strike the pavement marking material on the road surface can cause the marking spheres to roll along the pavement marking material in the direction of vehicle travel after initially striking the pavement marking material despite the tackiness of the pavement marking material. As the marking spheres roll, they pick up some of the pavement marking material on their surface, which prevents that portion of the marking sphere from reflecting light. To reduce marking sphere roll, the marking sphere dispenser may be positioned so that the marking spheres are ejected from the marking sphere dispensing device having a vector opposite the vector of vehicle travel. This opposing marking sphere vector cancels some or all of the relative velocity of the marking spheres and reduces rolling.
Marking sphere roll becomes more problematic by the continuing drive to apply the pavement marking materials and marking spheres at faster speeds so that the vehicles carrying the pavement marking devices minimally impact traffic conditions. As is understood, the faster the vehicle moves in one direction, the faster the marking spheres must travel in the opposite direction to reduce marking sphere roll. In this regard, some marking sphere dispensing devices impart a velocity to the marking spheres with pressurized fluid in a direction opposite to the direction of travel of the vehicle.